Which app or website tells us what price can we pay for a car ?

savingprivate

I'm new to cars and wanted to buy a car.
I will be paying full cash, no financing.

If I go to a dealer and ask them to show they would show some cars and give us a price.

How to negotiate a good deal with the car sales rep ?

Is their a website or a smart phone app which I can use and then feed in
the make and model with options and it can show me what would be the best price to pay ?

This way I know I haven't paid too much for a car and got ripped off during
the car buying process

Thanks in advance